1. Field of the Invention
Clocking devices currently available in time-measurement technology include a reference clock that gives a clocking signal. The choice of this signal normally depends on the field or range of measurement being considered, because the stability of the clocking signal is not homogeneous in relation to the use considered.
2. Description of the Prior Art
Thus, referring to FIG. 1, the current state of the art can be summarized by the stability curves of the best 5 MHz frequency quartz oscillators available in the market: these are the curve .circle.1 for a quartz oscillator marketed under the name 8600 OSA by the Oscilloquartz and the curves .circle.2 , .circle.3 and .circle.4 respectively relating to the option 4 CS and CS-HP5061A caesium clocks, marketed by Hewlett-Packard, and the CS 3210 caesium clock marketed by Oscilloquartz. In FIG. 1, the x-axis is graduated in integration or counting time .tau., i.e. the measuring range, and the y-axis is graduated according to the variance of relative frequency fluctuations expressed according to Allan's variance .delta.y(.tau.). It is observed that, for the short-term time measuring range, equivalent to a counting time .tau. ranging between 10.sup.-3 and 1000 s approximately, the best quartz oscillators give a frequency stability of .delta.y(.tau.) for which the frequency variance is substantially smaller than 10.sup.-12 : this variance increases very quickly and the frequency stability of the clocking signal deteriorates quickly for a counting time of more than 1000 seconds. On the contrary, the most efficient caesium clocks show a frequency variance that is far greater than 10.sup.-12 : hence, the clocking signal created by these clocks has a mediocre frequency stability for a counting time of less than 100 seconds, the frequency variance becoming, by contrast, smaller than 10.sup.-12 for a counting time that is greater than about 1000 seconds.
Thus, depending on the application and the counting time envisaged, it is necessary, for measuring time under optimum conditions of precision, to use either of the clocking devices (quartz oscillator or caesium clock) and, presently, there is no single frequency reference that can cover both the short-term measuring range (counting time 10.sup.-3 &lt;.tau.&lt;1000 s) and the long-term measuring range (.tau.&gt;1000 s) . The clocking device of substantially constant stability of the invention, for short-term and long-term time measurement, is aimed at removing the above-mentioned drawbacks by means of a single device, the optimum stability of which is smaller than or equal to 5.times.10.sup.-13 for a counting time of .tau.&gt;1 s.
Another object of the present invention is the application of a clocking device, used to obtain a clocking signal or reference signal having both the stability of the quartz oscillator for short-term measured counting times of less than 1000 seconds and the stability of the caesium clock for long-term measured counting times of more than 1000 seconds.